class_docstr: Main coreference resolution algorithm
code: def remove_singletons_clusters(self):
      remove_id = []
      for key, mentions in self.clusters.items():
          if len(mentions) == 1:
              remove_id.append(key)
              self.mention_to_cluster[key] = None
      for rem in remove_id:
          del self.clusters[rem]
  
imports: import os
  import spacy
  import numpy
  from neuralcoref.train.utils import PACKAGE_DIRECTORY, SIZE_SINGLE_IN
  from neuralcoref.train.compat import unicode_
  from neuralcoref.train.document import Document, MENTION_TYPE, NO_COREF_LIST
label_desc: Use image node_id 4 for calling the Coref obj's underlying member method code with example usage: obj.remove_singletons_clusters() without return types